Thin film resistive temperature detectors are a variety of temperature sensor used for detecting temperature in many applications, including but not limited to effluent or emissions from an engine. For example, such detectors may be used for detecting the exhaust gas temperature of an internal combustion engine. The exhaust gas temperature sensor may be part of an engine management system. Various operating parameters, such as fuel delivery, etc., may be adjusted based in part on a measured exhaust gas temperature.
Platinum metal film resistive temperature detectors are one particular variety of temperature sensor used for detecting effluent temperature. The platinum metal resistive element used in such temperature detectors is sensitive to environmental conditions. For example, a reducing atmosphere may cause migration of the platinum film of the resistive element from its substrate if oxygen in the surrounding atmosphere is below a threshold concentration. Significant loss of platinum from the resistive temperature detector resulting from decomposition or migration of the platinum may adversely affect the performance and life of the temperature detector.
The interior surfaces of a closed, or encapsulated, temperature sensor may react with the trapped oxygen in the closed environment, thereby reducing the oxygen concentration and leaving the platinum resistive element susceptible to damage from the resulting reducing environment. The volume of air which may be contained within the closed temperature sensor may be limited because too great an internal volume may insulate the resistive temperature detector element from the exterior of the sensor, greatly increasing the thermal response time and reducing the performance of the sensor. Because of the restrictions on the internal volume of the enclosure, even if the interior surfaces of the enclosure have been pre-oxidized prior to final assembly of the sensor, further oxidation of the interior surfaces and/or contaminates may still reduce the oxygen concentration leaving the platinum resistive element susceptible to damage.
Open temperatures probes, which do not provide a closed environment, are open to the outside atmosphere to allow oxygen exchange with the platinum film of the temperature detector in order to prevent the loss or migration of the metal film in the presence of a reducing atmosphere. While the open design may allow communication with the external atmosphere, the external atmosphere may not, necessarily, provide a sufficient oxygen concentration to prevent the loss of, or damage to, the thin film resistive element. Additionally, the open design may allow the entrance of contaminants, such as soot, dust, combustion byproducts, etc. Such contaminants may react with, attack, inhibit or otherwise negatively affect the substrate, the platinum film, the thermal response time, etc., of the temperature detector.